1. Field of the Invention
The invention relates to a method for the control of the preconduction currents supplied to laser diodes that are employed as light transmitters in the transmission stages of signal transmission systems with light wave guides. FNT (1) in technical literature described as "bias current"
2. The Prior Art
Digital wave guide transmission systems as a rule incorporate gallium-aluminum arsenide laser diodes as optical transmitters especially given high transmission velocities. Such laser diodes have a bend light/current characteristic curve which is illustrated, for example, in FIG. 2b. A current Ith occurring at the bend of the characteristic curve is hereby designated as a threshold current. A cut-in delay (2) as well as a rectifier effect for the modulation current flowing through the laser diode is produced by means of this threshold. Such a cut-in delay is not acceptable, particularly in the transmission of digital signals with high bit rates. Therefore the laser diode is usually supplied with a pre-conduction current Io which largely corresponds to the threshold current in its magnitude. Since, however, both the threshold current as well as the steepness of the subsequent characteristic curve branch of the laser diode depend on temperature and aging, a control for the threshold current is required. FNT (2) in technical literature described as "switch-on delay" FNT (3) in technical literature described as "external quantum efficiency"
A known, mean value, control has been constructed such that the absolute light output power of the laser diode is measured with the assistance of a photoreceiver and is compared with in index value which has been set. Given a deviation of the light output from the index value, the preconduction current of the laser diode is readjusted to control the means light output of the diode in such manner that the difference between the actual value and index value of the light output power is minimized or disappears. In this type of control, however, one cannot distinguish between a change in the threshold current and a change in the steepness of the laser diode characteristic curve. In this type of control, a decrease of the steepness would lead to an excessive increase of the preconduction current above the threshold current and of necessity diminish the possible modulation deviation. By so doing, the main light output then contains a steady light part that is too large which leads to an increased receiver noise and, moreover, reduces the part of the modulated light.
In another known control circuit with a peak value and a steady light control, both the preconduction current as well as the modulation current are tracked via two separate control loops. The control of the modulation current, not discussed further below is achieved by measuring the peak value of the light output. On the other hand, the control of the preconduction current, the magnitude proportional to the steady light part of the laser diode, is achieved by forming the difference between the peak value and mean value of the light output. However, the sensitivity of the control circuit is increased greatly as a result of forming the difference between two values having magnitudes of nearly the same size. Small amplification changes in the amplifiers required for the measurement of the peak and mean values can result in large or substantial output errors. This is particularly critical given high bit rates. At high bit rates, a great amplifier band-width is necessary resulting in only a small feed-back factor, and, additionally, the permissable deviation of the preconduction current from the desired value is very small in order to keep the time delay of cutting-in the laser diode very small.
The cited control circuits have a common disadvantage which derives from the measurement of the light of the laser diode. Namely, it is not the light emitted from the laser diode to the light wave guide segment which is measured, but, rather, the light emitted by the back mirror of the laser diode. Given a varying aging of the two mirrors, output errors of necessity are produced.
A further known control circuit is based on the temporal retardation between the cutting-in of the current pulse and the commencement of the stimulated emission in the laser diode. Prerequisite for this, however, is that the preconduction current is always smaller than the threshold current. This precondition, however, does not apply with respect to the transmission of digital signals with very high bit rates.